A simple model for transient eddy momentum fluxes in the upper troposphere

A simple model for transient eddy momentum fluxes is constructed based on t he hypothesis that planetary waves radiating from low-level baroclinic eddi es act effectively as a random forcing of the upper troposphere. The respon se of the upper troposphere is modeled by the barotropic, nondivergent vort icity equation, linearized about a zonally symmetric flow. Rayleigh frictio n and scale-selective damping are included to crudely represent the effect of vertical diffusion and nonlinear equilibration mechanisms; the associate d damping rates constitute the only tunable parameters of the model. A dist inguishing feature of the model is that wave-activity theory is used to spe cify the statistics of the random forcing as a function of the background f low and low-level eddy heat fluxes. The resulting model simulates the annua l cycle of momentum fluxes reasonably well, but tends to underestimate the eddy kinetic energy in midlatitudes. The results are interpreted in light o f an analytic solution to the linear response of a barotropic model to rand om forcing.